I may be the least experienced at this game but I know from playing WoW and FFXIV that a lot of times you just don't know all thing things available for end-game. With that said, feel free to recommend, add, or remove.
Guild Events - Check the Guilded Calendar or even better, work with your squad and create your own!
Legendary - Go through the main story on all 4 classes on both Republic and Imperial side for a special frame on your portrait (Flair) and title. You will also unlock all of your Heroic Moment abilities and get a shiny Guild Badge!
Gearing Up - get all of your toons to max gear and the best amplifiers. This includes getting bonus gear sets.
Operations and Flashpoints - Master level!
Warzones - PvP and you get commendations you can use for special gear!
World PvP - enjoy fighting
Ranked Warzones - Show off to the guild by getting ranked and working your way to #1
Galactic Starfighter - Pvp but with fighters. An amazing aspect of SWTOR that is completely different than your normal MMORPG.
Special Events - Things like Bounty Contract Week or Nightlife event (You can find info on these on the blog https://www.imperialarmy.org/blog )
Crafting - contribute to the guild or make some money selling on the GTN
Achievements - are you a completionist? Show off your hard work!
Holocrons - Increase your stats across your character's for maximum power!
Roleplaying (RP) - Just ask Nora and Dee, there are so many great things to do!
Galactic Strongholds - You can decorate, show off, and even set up a Hutta ball match!